The present invention relates generally to a method for suppressing laser speckle at an output of a short multimode optical fiber, and more particularly to a method of preserving beam pointing stability.
In general, laser beams possess high spectral purity. As a result of the spectral purity, as a laser beam is transmitted through the core of a multimode fiber and emerges from the fiber, it exhibits points of zero intensity, also known as a xe2x80x9cspeckle patternxe2x80x9d. The points of zero intensity are produced due to the fact that the fiber modes are coherent with one another but have random phasing. Because the laser beam emerging from the multimode fiber has points of zero intensity, it may be difficult to consistently deliver laser power onto small targets in a far field. Accordingly, the zero intensity phenomenon is a serious problem in multimode fiber-routed laser beam delivery applications.
It is further known that the speckle pattern of a laser beam emerging from a multimode fiber is sensitive to the fiber position. One known method of resolving the speckle pattern problem is to shake the fiber back and forth over long time durations on a millisecond time scale. However, in the case of a pulsed laser or in an apparatus where detector integration times are very short, the method of shaking the fiber is not desirable because no appreciable change in fiber position is possible on a sub-millisecond time scale.
Another method in use for reducing the points of zero intensity for a pulsed laser system is to incorporate a longer fiber length. Conventional multimode fibers comprise an optical core surrounded by cladding, as is well known in fiber optic technology. Light is transmitted through the core of a multimode fiber in light groups known as modes. A multimode fiber is capable of supporting light for transmission in plural groups of core modes. Higher order modes bounce back and forth as they travel the length of the fiber. Accordingly, the higher order modes are time delayed with respect to the lower order modes. If the time delay is longer than the temporal coherence length of the laser, the core modes add in intensity as they emerge from the fiber, resulting in a suppressed speckle pattern.
When incorporating the method of using a longer multimode fiber length for a pulsed system, in order to calculate the proper length of the fiber necessary a typical transform limited pulse duration, xcfx84p, of 7 nsec is launched into the fiber. A mode delay, xcfx84md is calculated between the highest and lowest modes in a step index core clad optical fiber length L with N.A. (numerical aperture) of 0.3 and an index n of 2.4 of:                                           τ            md                    L                =                                                            (                                  N                  .                  A                  .                                )                            2                                      2              ⁢                              xe2x80x83                            ⁢              cn                                =                      60            ⁢                          xe2x80x83                        ⁢            psec            ⁢                          /                        ⁢            m                                              (Equation  1)            
The index n of 2.4 is a typical value of an infra-red (IR) transmitting As2S3 fiber, and c is the speed of light in a vacuum environment. Accordingly, a fiber length of 1.0 meter would have the highest mode delayed by 60 psec relative to the lowest mode, LP01.
The speckle pattern which emerges from a laser beam entering a multimode fiber generally exhibits a 100% root mean square (RMS) fluctuation in amplitude. This arises because the fiber modes are all coherent with one another but have random relative phasing. In order to calculate the RMS amplitude noise between the various delayed modal components of the beam, one notes that for xcfx84md greater than xcfx84p                                           (                                          Δ                ⁢                                  xe2x80x83                                ⁢                I                            I                        )                    ⁢                      xe2x80x83                    ⁢          RMS                =                              τ            p                                2            ⁢                          xe2x80x83                        ⁢                          τ              md                                                          (Equation  2)            
where I is the intensity in space across the emitting aperture and the brackets are taken to mean a spatial average over this aperture. The factor of two arises from the two polarization states. For example, to produce a 25% RMS fluctuation in the beam, one needs a maximum mode delay xcfx84md of about 7 nsec, and accordingly a fiber length of at least 233 m. A mode delay xcfx84md of 7 nsec is the maximum mode delay and should therefore be viewed as a best-case estimate.
Although this length is attainable in silica fibers with reasonable optical losses for visible and near-IR lasers, this is an unrealistically long length for current mid-IR transmitting fibers such as those based on As2S3. Furthermore, in pulsed laser systems, the pulse emerging from such a long fiber is considerably longer, by nearly a factor of two, than the input pulse, which may be a disadvantage in a laser ranging or plasma generation system.
An alternative known method for reducing the points of zero intensity emerging from a multimode fiber is to increase the laser bandwidth incorporating either active or passive means. Instead of using a longer fiber length, the temporal coherence length xcfx84c is reduced. In this method, the amount of speckle reduction is calculated by replacing xcfx84p with xcfx84c in Equation 2. The amount of spectral bandwidth xcfx84cxe2x88x92 needed in a one meter length of the above fiber to achieve a 25% RMS speckle amplitude is 33 GHz., wherein xcfx84cxcx9c0.25*2*60=30 psec. This spectral broadening could be produced by means of an electro-optic device for visible and near IR applications. However, most commercially available electro-optic modulators are inefficient above 1 GHz.. Accordingly, in the mid IR region, such a large broadening is not possible with current commercially available devices.
It is therefore an object of the present invention to reduce the speckle pattern of a laser beam emerging from a multimode fiber without significantly increasing the pulse width or spectral bandwidth.
It is a further object of the invention to provide a method and apparatus for suppressing laser speckle at an output of a short multimode fiber by changing the launch angle by at least one divergence in time.
In accordance with the invention, these and other objectives are achieved by providing a means for translating a laser beam in a conical shape around a launch lens for projecting into a multimode fiber thereby reducing points of zero intensity emerging from the fiber.